According to basic electromagnetic theory, a time-varying current in a wire will produce an associated time-varying electromagnetic field around the wire, and a time varying electromagnetic field near a wire will produce an associated current in that wire. This is the basic premise of all antennas. Since the power lines in a home are essentially a collection of wires, they may potentially be used as antennas. Additionally, the antenna reciprocity theorem tells us that any antenna is an equally good transmitter or receiver.
The use of a power line as an RF antenna has been explored in various contexts as far back as the 1920's. Several patents describe various forms of a line cord antenna, whereby a receiver is coupled to the power line to receive high-powered broadcasts originating outside the home from TV and radio stations. Power lines have also been examined as transmitting antennas, which are further categorized for regulatory purposes as intentional or unintentional radiators of electromagnetic signals. As an intentional radiator, power lines have been used to distribute AM radio broadcast signals over the main power distribution grid. Within a home or other structure, power lines operating as intentional radiators have been used to distribute RF signals at up to 20 MHz for an indoor localization system, for a cordless phone system transmitter, and for an in-home video distribution system transmitter. As unintentional radiators, power lines unintentionally leak radiated RF fields from broadband over power line (BPL) signals. BPL signals are intended to be confined to the power lines, but nonetheless unintentionally radiate RF energy since power lines are not designed to function as radio frequency transmission lines. The development of power line tags (PL-Tags), disclosed in United States Patent Application Publication US 2010/0109842, Ser. No. 12/560,099, showed that the power line could be used as a receiving antenna for detecting low-frequency resonant RF energy from power line activated passive RFID tags.
A consistently emerging theme in ubiquitous computing for the home is that while it is possible to think of the home as a sensor-rich environment, there are practical roadblocks to widespread deployment. Though the research community and others have begun to define more and more compelling applications of sensor enabled environments, many remaining obstacles hinder the adoption of in-home sensors. Power requirements are a frequent source of concern for wireless sensor networks. Wireless sensors are appealing because they can be placed in a wide variety of locations throughout a home, but the high power consumption of most wireless communication technologies limit the sensor's battery life to an undesirably short operating period. Deploying tens or hundreds of wireless sensors in a home will result in an undesirably high maintenance cost due to the battery cost as well as the labor cost required to check and replace the sensor batteries.
Accordingly, there is a need for a wireless communication system that permits building-wide coverage from wireless sensors, while consuming as little battery power as possible from the wireless sensor nodes. The present invention takes advantage of the transduction properties of existing electrical power line infrastructure to allow wireless sensors to operate with very low battery power consumption compared to competing wireless communication approaches.